For mashed potato topping

Directions

Special equipment: a wide 3-quart heavy flameproof casserole (about 2 inches deep; not glass); a potato ricer or a food mill fitted with medium disk Prepare filling: Blanch onions in a 2- to 3-quart pot of boiling salted water 1 minute, then transfer with a slotted spoon to a bowl of cold water to stop cooking.

Drain onions and peel, trimming root ends with a paring knife.

Wash leek slices in a bowl of water, agitating them, then lift out and drain in a medium-mesh sieve.

Put lamb and 3 tablespoons flour in a sealable plastic bag, then seal bag and shake to coat lamb.

Melt 2 tablespoons butter in casserole dish over moderately high heat until foam subsides, then brown half of lamb, turning, about 6 minutes.

Add 1 tablespoon garlic and cook, stirring, 2 minutes.

Transfer browned lamb and garlic to a plate with a slotted spoon and repeat with remaining lamb and tablespoon garlic (do not add more butter).

Add wine to casserole dish and deglaze by boiling over high heat, stirring and scraping up brown bits, 1 minute, then stir in tomato paste and boil, stirring, until liquid is reduced by half, about 5 minutes.